The present invention relates to a process for the preparation of 13-cis isomer of Vitamin A acid, commonly known as isotretinoin, in a single step.
Isotretinoin (13-cis retinoic acid) belongs to a family of Vitamin A (retinol) related compounds. It inhibits sebaceous gland function and keratinization and is used for the treatment of dermatological diseases like acne. It is extremely effective in very severe and nodulocystic acne and prevents scarring. More recently, isotretinoin has also been evaluated for its potential use in certain cancerous conditions.
Structurally, isotretinoin is a highly conjugated molecule consisting of a substituted cyclohexene moiety and a nine-carbon polyene side chain with a terminal carboxy group. All but one of the double bonds (C-13 double bond) in the side chain are trans and it is the stereospecific construction of this polyene side chain which has challenged synthetic organic chemist for the last almost three decades. Commercially and readily available xcex2-ionone has been conveniently used for the construction of the cyclohexene part of isotretinoin. The synthetic prior art approaches for the construction of the polyene side chain are summarized below.
In general, a convergent approach, involving stereospecific coupling of the appropriate C15 (synthesized from xcex2-ionone) and C5 synthons, has been utilized. (however, a linear sequence comprising of seven steps, starting from xcex2-ionone, has also been described; J Org. Chem. 54, 2620-2628, 1989). For example, Patternden and Weedon, J. Chem. Soc.(C), 1984-97 (1968) have disclosed a procedure for the preparation of 13-cis retinoic acid by reacting a C15-triarylphosphonium salt (Wittig salt) and a C5-butenolide in diethylether to produce an isomeric mixture (of the cis and trans isomers at C-11 double bond) of 13-cis retinoic acid in 66-75% yield; the desired 11-trans-13-cis content is only about 36% and the rest being the corresponding 11,13-di cis isomer. Selective isomerization of the 11-cis double bond in the presence of 13-cis double bond proved extremely difficult to accomplish. A great deal of effort has been directed to affect selective isomerization of 11-cis double bond (without isomerizing the 13-cis double bond) in 11,13-di cis retinoic acid. The methods include photoisomerisation by using either iodine (J. Chem. Soc. (C) 1982, 1968), transition metal catalysts (U.S. Pat. No. 4,556,518) or photosensitizers such as erythrosin B, rose Bengal etc. (U.S. Pat. No. 5,424,465). These processes suffer from the following limitations and for various reasons are not suitable for commercial production of isotretinoin. For example, the process for selective photoisomerization using iodine under diffused light is extremely difficult to accomplish without affecting the 13-cis double bond. This results in the generation of all trans retinoic acid (tretinoin) as a major impurity in isotretinoin produced by this process. Although, U.S. Pat. No. 5,424,465 describe that use of photosensitizers enhances selectivity of photoisomerisation of C11-cis double bond, no data, however, is provided for the extent of tretinoin formation in this process.
The use of the palladium catalysts, as described in U.S. Pat. No. 4,556,518, could potentially lead to the contamination of the desired isotretinoin with traces of transition metals and thereby might lead to problems with the stability. In addition, the process involves an elaborate extraction procedure for the work-up.
U.S. Pat. No. 4,916,250 describes a process involving use of a phosphonate ester (as a C15 synthon), which is first generated in several steps starting from xcex2-ionone. The phosphonate ester is then reacted with 5-hydroxy-4-methyl-2- (5H)-furanone (C5 synthon) to afford isotretinoin. Although this approach does not involve the cumbersome photoisomerization step, it is uneconomical at a commercial manufacturing scale because of the large number of steps.
Cainelli et al, Gazz. Chim. Ital, 103, 117-125 (1973) reported the synthesis of isotretinoin by reacting a dienolate of sodium 3,3-dimethyl acrylate (C5-synthon) with xcex2-ionylideneacetaldeyhyde (C15 synthons) at xe2x88x9278xc2x0 C. for twelve hours to give a hydroxy acid intermediate. The hydroxy acid intermediate on conversion to intermediate lactone and subsequent treatment with base afforded isotretinoin. This approach suffers from the following limitations two different bases (sodium hydride and lithium diisopropylamide) are required and moreover generation of dienolate requires maintaining low temperatures (xe2x88x9278xc2x0 C.) for extended periods of time, which would entail very high energy costs at the commercial scale. Furthermore, the purification of the intermediate lactone by preparative High Performance Liquid Chromatography, as suggested, is not commercially feasible.
It is an object of the present invention to solve the problems associated with the prior art and to provide an efficient method for the synthesis of isotretinoin of high purity in one single step (stereospecific coupling of C15 and C5 synthons) using conditions which are convenient to operate on a commercial scale.
It is a further object of the present invention to provide a process which affords isotretinoin while controlling the levels of tretinoin to  less than 0.1%. Various pharmacopoeias have prescribed a 1-2% limit of this impurity in isotretinoin.
The present invention is directed to a process for the preparation of isotretinoin, which comprises the condensation of dienolate of methyl-3,3-dimethylacrylate of Formula I: 
with xcex2-ionylideneacetaldehyde of Formula II: 
in a suitable solvent at (I) xe2x88x9260xc2x0 C. to xe2x88x9280xc2x0 C. for 1-2 hours and (ii) 25xc2x0 C.-45xc2x0 C. for 1-24 hours, followed by aqueous acidic work up to give isotretinoin in a single step.
The condensation reaction proceeds via the formation of the intermediate lactone of Formula III: 
which is not isolated. Lactonization results in the release of a methoxide ion which in turn opens the lactone to afford isotretinoin (as carboxylate salt); the reaction of methoxide and lactone is facilitated by higher temperatures (25-45xc2x0 C.) and by carrying the reaction for longer time. Aqueous acidic work up thus produces isotretinoin in one single step starting from xcex2-ionylidene acetaldehyde.
Generally, the initial condensation of dienolate of methyl-3,3-dimethyl acrylate of Formula I shown above with xcex2-ionylidene-acetaldehyde of Formula-II above is carried out at xe2x88x9260xc2x0 C. to xe2x88x9280xc2x0 C. for 1-2 hours. Preferably, it is carried out at xe2x88x9265xc2x0 C. to xe2x88x9275xc2x0 C. Temperature is later raised to about 25xc2x0 C.-45xc2x0 C., preferably between 30-40xc2x0 C. and is maintained for 1-24 hours and the progress of the reaction is monitored. Suitable solvents include tetrahydrofuran, 1,4-dioxane, hexanes, diisopropyl ether, hexamethyl-phosphoramide, tetramethylurea, and mixtures thereof. Tetrahydrofuran is a preferred solvent.
Aqueous acidic work up involves the adjustment of pH with mineral acids and extraction with organic solvents. Acids may include hydrochloric acid, sulfuric acid, and phosphoric acid. Sulfuric acid being the preferred acid. Any organic solvent may be used for extraction and such solvents are known to a person of ordinary skill in the art and include: water-immiscible solvents, such as chloroform, dichloromethane, 1,2-dichloroethane, hexane, toluene, ethyl acetate and the like.
Other features of the invention will become apparent in the course of the following description of exemplary embodiment, which is given for illustration of the invention, and are not intended to be limiting thereof.